1. Field of the Invention
The present invention relates to a thin film type monolithic semiconductor device which has a plurality of thin film transistors (TFTs). The TFTs fabricated in the present invention are formed both on insulating substrates such as glass or the like and on semiconductor substrates such as single crystal silicon or the like. More particularly, the present invention relates to a semiconductor circuit which has a low speed operating matrix circuit such as a monolithic active matrix type circuit (which is used in a liquid crystal display or the like) and a high speed operating peripheral circuit to drive the matrix circuit.
2. Description of the Prior Art
In recent years, research has been made on semiconductor devices of insulated gate type, the devices having a thin film-like active layer (which is referred to as an active layer) on an insulating substrate. In particular, efforts are concentrated on studying thin film gate type transistors, or so-called thin film transistors (TFTs). The TFTs are formed on a transparent insulating film, and are used for the control of each pixel and in a driving circuit in a display which is formed of a liquid crystal or the like and which has a matrix structure.
Examples of thin film semiconductors which constitute TFTs include amorphous silicon semiconductors and crystalline semiconductors which are crystallized by heating or laser light irradiation of the amorphous silicon semiconductors. The TFTs using these amorphous silicon thin film and crystalline silicon thin film are referred to as amorphous silicon TFTs and crystalline TFTs.
Generally, the field mobility of semiconductors in an amorphous state is small, and therefore cannot be used in TFTs which are required to be operated at a high speed. Therefore, research and development has been carried out in recent years on crystalline TFTs for the fabrication of circuits which have higher performance.
Crystalline semiconductors have large field mobilities, and therefore can be operated at high speed. Since NMOS TFTs and PMOS TFTs are obtained with the crystalline silicon in the same manner, a CMOS circuit can be formed. For example, in an active matrix type liquid crystal display device, display devices with a monolithic structure in which both an active matrix type part and a peripheral circuit (drivers or the like) are constituted with a CMOS crystalline TFTs are known.
FIG. 3 shows a block diagram of a monolithic active matrix circuit used in a liquid crystal display. In a structure shown in FIG. 3, a column decoder 1 and a line decoder 2 are provided on a substrate 7 as a peripheral driver circuit. Further, in a matrix area 3 in which a plurality of pixels are arranged in a matrix configuration, a plurality of pixel circuits 4 which comprise transistors and capacitors are formed so that the matrix area and the peripheral circuit are connected with wires 5 and 6. The TFTs used in the peripheral circuit are required to be operated at a high speed while the TFTs used in the pixel circuit are required to have low current characteristics. These two characteristics are inconsistent to each other in terms of physics. However, the TFTs used in the peripheral circuit and the TFTs used in the pixel circuit are demanded to be formed on the same substrate at the same time.
However, the TFTs fabricated in the same process all exhibit the same characteristics. For example, means of crystallization (so-called laser anneal) can be used to obtain a crystalline silicon. However, in a silicon which has been crystallized by laser crystallization, the TFTs in the matrix area and TFTs in the peripheral driving circuit area exhibit the same characteristics. Therefore, the low leak current characteristics demanded of the pixel circuit and the high field mobility characteristics demanded of the peripheral circuit can coexist with great difficulty. The present invention is intended to solve such a difficult problem.